Drainage & Hydrology
Detention Pond Sizing
Preliminary detention storage volume from peak inflow, allowable outflow, and storm duration using the simplified triangular hydrograph method.
Detention Pond Sizing
This calculator estimates the storage volume a detention pond needs to reduce post-development peak runoff down to an allowable release rate, using the simplified triangular hydrograph approach common for preliminary sizing on small to medium sites. It pairs naturally with this site's Rational Method and Time of Concentration calculators, which supply the peak inflow and storm duration inputs.
The method assumes the inflow hydrograph rises and falls linearly over the storm duration while the outflow structure releases at a constant allowable rate. The required storage volume is the excess between the two, integrated over the storm — a quick, conservative first estimate before full hydrograph routing.
V_req = 0.5 · (Q_i − Q_o) · t_d · 60 [ft³, cfs, minutes]
A site with a post-development peak inflow of 12 cfs must release no more than the 4 cfs pre-development peak, over a 20-minute critical storm duration. The required storage is V_req = 0.5 × (12 − 4) × 20 × 60 = 4,800 ft³, or about 0.110 acre-feet — a 67% reduction in peak flow that the pond's storage and outlet structure must be sized to achieve.
Detention Pond Sizing Calculator
Simplified triangular hydrograph · imperial units